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Impression Materials

Dental Material



  1. Impression when undercuts are present
  2. Partial denture with clasps
  3. Used even in cases of excess salivation
  4. Preliminary Impressions for complete denture
  5. Duplicating Models
  6. Mouth Guards
  7. Orthodontic


Ester salts of alginic acid Sodium or Potassium or Triethanlolamine aginate – 15%

Dissolves in water and reacts with calcium ions

Calcium Sulphate (Reactor) – 16%

Reacts with alginate (Na or K) and forms insoluble calcium alginate

Sodium Phosphate (Retarder) – 2%

Reacts preferentially with calcium sulphate to reduce the rate of reaction

Zinc Oxide – 4%


Potassium titanium fluoride – 3%

Gypsum hardener

Diatomaceous earth – 60%


Organic glycol

Coats powder particles in dust-free alginates

Colouring & Flavouring agent


It is a chemical reaction. Insoluble calcium alginate gel is formed when soluble alginate reacts with calcium sulphate.

To delay this reaction to occur in patients mouth, a retarder is added e.g. Na or K phosphate, oxalate or carbonate. By adding the retarder, calcium sulphate reacts with retarder first to increase the working time. The formed product may be:

  1. Tetrasodium pyrophosphate
  2. Sodium Tripolyphosphate
  3. Trisodium phosphate

2 reactions occur during setting:

Na3PO4 + 3CaSO4 -----→ Ca3(PO)4 + 3Na2SO4

Sodium alginate + CaSO4 + H2O ---→ Ca alginate + Na2SO4


Sodium phosphate reacts with calcium Sulphate give working time for the mix.

After Sodium Phosphate has reacted, Calcium Sulphate reacts with sodium alginate to form insoluble calcium alginate which is a gel when combined with water.

To control the gelation time,

Colder the water, longer is the gelation time

Warmer the water, shorter is the gelation time.

If the water/powder ratio is altered, the properties of alginate are hampered.


Clean flexible rubber bowl, spatula, Figure of 8. Swapping alginate against the wall of rubber bowl, make sure the mix is smooth and homogenous.

Figure of 8 is used because:

Promotes dissolution by mixing un-dissolved algin with dissolved algin

Remove most air bubbles

Mixing Time:

Fast set alginate – 45 seconds

Normal set alginate – 60 seconds

Over-mixing – Reduced working time, Reduced final strength due to destruction of gel fibrin.

Under-mixing – Reduced strength, Inadequate wetting, lack of homogeneity poor recording due to grainy mix  

Working Time:

Fast set alginate – 1.15 minutes

Normal set alginate – 2 minutes

Properties of Alginate Hydrocolloid Impression Materials

According to ADA specification 18:

Working Time:

Fast Setting: 1.2 to 2 minutes

Normal setting: 2 to 4.5 minutes

Setting Time:

Ranges from 1 to 5 minutes

For fast setting: 1 to 2 minutes

Normal Setting: 2 to 4.5 minutes


14% stress of 1000g/cm2. Lower W/P ratio results in lower flexibility.


Compressive: 5000 to 8000 g/cm2

Tear: 350 to 700 g/cm2 

Biological Properties:

Due to airborne silica particles on fluffing of the alginate packet, some manufacturers are now supplying dust-free alginates. 

Dimensional Stability:

Alginate Impressions loses water by evaporation and shrinks (Syneresis) and swells when kept in water (Imbibition). Hence, cast must be immediately poured after impression taking. Keeping in humid atmosphere of 100% relative humidity, least dimensional changes can be expected.

Words to remember:

Imbibition: Gain of water by immersion in water.

Syneresis: Loss of water by evaporation or exuding fluid


Good surface detail even in excess saliva

Easy to mix and manipulate

Low cost

Minimum requirement of armamentarium

Accuracy if handled properly

Comfortable to patient


Flexibility of set impression 


Cannot be corrected

Tissue compression is not achieved

Inaccurate for crown and bridge impressions

Distortion may occur easily

Metal dyes cannot be prepared.

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